The invention relates to a method for the production of a pressure pipe of a tractive solenoid for a hydraulic valve, and to a pressure pipe of a tractive solenoid for a hydraulic valve.
A pressure pipe of this kind is known from DE 197 07 587 A1 and is a component of a pressure-tight tractive solenoid, which, in addition to the pressure pipe, has a coil for actuating an armature, which is contained so that it can move axially in a receptacle of the pressure pipe, with the coil encompassing the pressure pipe. The pressure pipe includes a pole piece that can be screwed to a valve body by means of an internal thread, a nonmagnetic intermediate piece, and a tubular piece adjoining this, which is closed at the end oriented away from the pole piece by means of a component serving as a stroke limiter. The pole piece, the intermediate piece, the tubular piece, and the stroke limiter delimit the receptacle for the armature that cooperates with the coil. The armature is connected to a tappet, which extends through the pole piece in the axial direction and serves to actuate a valve slider of the hydraulic valve. The nonmagnetic intermediate piece serves to divert the magnetic flux into the armature.
The nonmagnetic intermediate piece is usually built into the pressure pipe through the use of a manufacturing method such as a bronze weld cladding process or the soldering-in of austenitic steel. When methods like these are used, it is necessary for the pressure pipe to be finished on an inner wall delimiting the receptacle through the use of a material-removing cutting process in order to produce a sliding surface for the armature. This is disadvantageous insofar as the material-removing cutting process not only lengthens manufacturing time but also increases production costs.
It is also known to produce the nonmagnetic intermediate piece or the nonmagnetic zone through partial structural transformation of the work piece that constitutes the pressure pipe. But it is disadvantageous that the structural transformation requires a large technical setup as well as a very sensitive control of the structural transformation apparatus in order to produce an intermediate piece with the desired structure.
In the above-mentioned methods for producing the nonmagnetic zone, because of the temperatures that occur, the armature and other components contained in the receptacle such as antistick washers and the tappet are inserted into the receptacle only after the joining process and the finishing of the connecting region between the pole piece and the intermediate piece and between the intermediate piece and the tubular piece. Only then it is the receptacle closed by means of the stroke limiter. This closing is usually produced by means of a crimping process.
A disadvantage to this method is that the stroke limiter is joined to the pressure pipe by means of a crimping process. The internal compression stress that occurs in the receptacle region changes the position of the stroke limiter so that a pressure-tight, sealed operation and a fixed seat are no longer assured. This is primarily due to wear on the edges inside the crimp with each change in pressure.
Particularly in pressure pipes with an integrated path-measuring device for measuring the path of a slider, a position change of the stroke limiter is disadvantageous because the position change shifts the zero point and consequently distorts the measuring results.
Another disadvantage of the method is that the stroke limiter is physically separate from the pressure pipe so that these two components must always be joined by means of a joining step.